Connect Chapter 5 - Linkage and Recomgination- mapping

A double crossover between two genes could involve ______, ______ , or _____ chromatids.

- 4 - 3 - 2

What two things are required, so that two homologous chromosomes in a tetrad experience tension when properly connected to spindle fibers?

- Chromosomes must be connected to fibers from opposite poles. - Chromosomes must be joined by a physical link.

Chromosome mapping can be achieved by performing multiple two-point crosses to determine the distance and order of genes on a chromosome. Choose the two limitations of two-point crosses in chromosome mapping.

- The distances calculated between different pairs of genes don't always add up. - The order of genes can be difficult to determine if some gene pairs are very close together.

Which two situations would result in a recombination frequency of 50% between two genes?

- The genes are located very far apart on the same chromosome. - The genes are located on two different chromosomes.

In the following equation, C stands for the _______ of _______.C = observed number of double crossovers/expected number of double crossovers

- coefficient - coincidence

Compared to two-factor testcrosses, three-factor testcrosses can provide additional information about ____.

- gene order - map distance

The transmission of _______ that are close to each other on the same chromosome violates Mendel's law of _________ assortment.

- genes - independent

Distance on a chromosome is generally measured in _____.

- map units - centiMorgans

Using a testcross, geneticists can analyze gene recombination by crossing an individual that is homozygous for the _______ alleles of two genes with a doubly _______ individual.

- recessive - heterozygous

The fraction of _____ in a testcross can be used to establish the degree of _____ between two genes.

- recombinants - linkage

You are performing a cross involving two X-linked genes in Drosophila. The F1 females are w+ y+/w y. Which of the following F2 males will be of the parental type?

- w+ y+/Y - w y/Y

If the frequency of double crossovers observed in the progeny of a three-point cross equals expectations (as calculated by multiplying the recombination frequencies of single crossovers in regions 1 and 2), then interference is _____.

0

A chromosome contains three genes in the following order: a - b - c. The distance between gene a and b is 5 mu. The distance between b and c is 3 mu. What is the expected likelihood of a double crossover?

0.0015

What is the value for interference if the coefficient of coincidence is 0.3?

0.7

Click and drag on elements in order Consider a three-point cross involving the genes vg, pr and b, which are ordered on the genetic map of Drosophila as depicted in the image. Order parental and recombinant classes below according to the frequency with which they are produced in the testcross. Start with the most frequent class at the top.

1. Parental classes 2. Recombinant classes with a single crossover in region 1 3. Recombinant classes with a single crossover in region 2 4. Recombinant classes with a double crossover in regions 1 and 2

In Drosophila, the X-linked genes for eye color (w) and body color (y) recombine with a frequency of 1.1% (as observed in the F2 progeny of a testcross involving those genes). That means that the w and y genes are

1.1 map units apart.

Two genes located on the same chromosome in fruit flies are 12 map units apart. If a scientist performs a testcross, at what frequency would the testcross progeny have the alleles of these genes in the recombinant configuration?

12%

A scientist collects data from a two-factor cross with a null hypothesis that 4 different types of offspring will be produced in a 1:1:1:1 ratio. After calculating the chi-square value using the chi-square test for goodness of fit, the scientist must look up the corresponding p value. How many degrees of freedom will she use when using the chi-square table?

3

The meiosis shown on this image will produce ______.

4 parental gametes

A geneticist is performing a chi-square test for goodness of fit on data from a genetic cross where 6 genotypes are expected in a 1:1:1:1:1:1 ratio. How many degrees of freedom does this test have?

5

A geneticist is performing a chi-square test for goodness of fit on data from a genetic cross where 6 genotypes are expected in a 1:1:1:1:1:1 ratio. How many degrees of freedom does this test have? Multiple choice question.

5

How many map units apart do two genes have to be to follow the law of independent assortment?

50 m.u.

The average fraction of recombinant gametes produced by a double crossover (DCO) is ____.

50%

Based on Mendel's law of independent assortment, what phenotypic ratio should the offspring of a two-factor cross between two dihybrids show, assuming that the two genes control different traits?

9:3:3:1

Which of the following statements accurately describes the length of chromosomes?

A chromosome can be more than 50 m.u. long.

In fruit flies, two X-linked genes specify the color of eyes and the color of the body as follows: the white gene has the alleles: w+ (dominant), which specifies red eyes, and w (recessive), which confers white eyes; the yellow gene has the allele y+ (dominant) which specifies a brown body and the allele y (recessive) which specifies a yellow body. What will be the phenotype of a fly with the genotype w y/w y+?

A female fly with white eyes and a brown body

In fruit flies, two X-linked genes specify the color of eyes and the color of the body as follows: the whitegene has the alleles: w+ (dominant), which specifies red eyes, and w (recessive), which confers white eyes; the yellow gene has the allele y+ (dominant) which specifies a brown body and the allele y (recessive) which specifies a yellow body. What will be the phenotype of a fly with the genotype w y/w y+?

A female fly with white eyes and a brown body

When performing a statistical test, such as the Chi-Square test, what p-value indicates that the data differ significantly from that expected if the null hypothesis is true?

A p-value of 0.05 or lower

Why are three-factor crosses preferred to two-factor crosses when mapping genes?

A three-factor cross can provide more accurate map distances.

What type of data is used in the chi-square test when applied to genetic crosses?

Actual numbers of offspring recorded during the test

If a female fruit fly with mutant white eyes and a wild-type brown body (w y+/w y+) is crossed with a male fruit fly that has wild-type red eyes and a mutant yellow body (w+ y/Y), what will the male offspring look like?

All male offspring will have white eyes and brown bodies.

You perform a dihybrid cross involving two linked genes where one pure-breeding parent has both dominant phenotypes and the other pure-breeding parent has both recessive phenotypes. What would you expect expect to observe in the F2?

An altered ratio of phenotypes where the parental classes will increase at the expense of the recombinant classes

A scientist is studying three genes, A, B, and C, located on the same chromosome. The scientist determined the following recombination frequencies between those genes: A and C: 16.4%; B and C: 8.6%. If the order of the genes on the chromosomes was A - C - B, what would be the expected recombination frequency between genes A and B?

Approximately 25%

Which letter on the diagram indicates a meiosis that will produce both recombinant and parental gametes?

B

Based on the data shown in the image depicting a three-point testcross, a scientist calculates that the distance between vg and b is 17.7 m.u., the distance between vg and pr is 12.3 m.u., and the distance between b and pr is 6.4 m.u. The scientist deduces that the pr gene is located between the vg and bgenes. Why?

Because the recombination frequency between vg and b is largest

How did scientists obtain evidence that tied genetic recombination to crossing over?

By using chromosomes with abnormalities that allowed them to track segment exchange between chromosomes during meiosis

A scientist is studying three genes, A, B, and C, located on the same chromosome. The scientist determined the following recombination frequencies between those genes: A and B: 23.1%; A and C: 8.6%; B and C: 31.4%. According to these results, which of the following could be the order of those genes on the chromosome?

C - A - B

Which phenomenon is thought to ensure that every pair of homologous chromosomes undergoes at least one crossover during meiosis?

Chromosomal interference

What is the name of the process in diploid eukaryotic species where homologous chromosomes exchange pieces with each other?

Crossing-over

How do double crossovers affect the calculation of physical distances between genes based on recombination frequencies?

DCOs can result in an underestimation of the physical distance.

What can affect the outcome of a cross, causing scientists to reject the independent assortment hypothesis, even though it may be correct?

Decreased viability of particular genotypes

Which statement accurately describes the degree of linkage between pairs of genes on the same chromosome?

Different pairs of genes on the same chromosome will exhibit different degrees of linkage, and some will assort independently.

Based on the data shown in the image depicting a three-point testcross for three X-linked genes, which gene is located between the other two? (Note that before analyzing the data, we do not know the gene order, and so in the diagram the gene order is arbitrary.)

Gene pr is located between vg and b.

Some pairs of genes on the X chromosome exhibit genetic linkage, while other pairs of genes appear to assort independently. What explains this observation?

Genes can assort independently if crossing-over occurs between them during meiosis.

What is the name of the phenomenon in which alleles of genes located together on a chromosome are usually transmitted together?

Genetic linkage

What is the name of the process that involves crossing over during meiosis, generating a new combination of alleles?

Genetic recombination

You perform a Chi-Square test and obtain a p-value lower than 0.01. What does that mean?

If the null hypothesis is true, there is less than 1 chance in 100 that results this different from those expected could be obtained by chance.

If two genes located on the X chromosome are linked, which of the following is always true?

In a dihybrid cross, a larger proportion of the offspring will be of the parental type.

Which of the following is true about the inheritance of X-linked genes?

In many cases when two or more genes are explored, their alleles do not assort independently.

Consider a cross between a female Drosophila carrying the two X-chromosomes in this image and a male. What would the X chromosome of a car+ Bar/Y male look like?

It would have an additional Y fragment on one end and a discontinuity on the other end.

How many genes does a typical human chromosome contain?

Many hundreds

Consider a three-point cross involving the genes vg, pr and b, which are ordered on the genetic map of Drosophila as depicted in the image. Which of the following recombinants should be present with the highest frequency?

Recombinant classes with a single crossover in region 1

What happens when a homologous chromosome pair does not undergo even a single crossover during meiosis?

The chromosomes often segregate improperly.

How would we interpret a p value of 0.20 resulting from a chi-square test for goodness of fit performed with data from a two-factor cross?

The null hypothesis cannot be rejected.

The results of the cross depicted in the image suggest that the white and yellow genes are linked. How would the results vary if the genes were independently assorting?

The number of recombinant genotypes and the number of parental genotypes will be found in roughly equal proportions.

During meiosis I, paired homologous chromosome must be connected to spindle fibers from opposite poles. Which of the following is a key element that ensures that this happens?

The physical tension exerted by the spindle when chromosomes are properly connected

You perform two crosses to test the same null hypothesis. In the first cross, you obtain a total of 125 offspring for analysis. In the second cross, you obtain 1,200 offspring for analysis. Which of the two datasets is better suited for chi-square analysis?

The second cross is better suited, because the larger the sample size, the less likely that deviations from the expected results happened by chance.

A three-point testcross involving three linked genes results in 8 progeny classes: 2 are parental types and 6 are recombinant types. Which of the recombinant classes correspond to offspring produced by gametes with double-crossover chromosomes?

The two recombinant classes that are observed with the lowest frequency

In a three-factor cross, what is the result of complete interference?

There are no double crossovers.

A scientist crosses two true-breeding pea plants which differ in two traits. All of the thousands of hybrid offspring look like one of the parents. The scientist allows the hybrids to cross and she observes that 75% of the F2 offspring are parental types, and 25% are recombinant types. What can be concluded about the linkage of the genes that specify the two traits?

They are linked because the number of parental types exceeds 10/16 and the number of recombinant types is less than 6/16.

What is the goal of genetic mapping?

To assign genes to positions on a chromosome

Can we measure the full length of a chromosome in m.u., and if so, how?

Yes, by adding many short distances between many pairs of genes

A testcross is a cross between _____.

a double heterozygote and a double recessive homozygote

All the genes located on a particular chromosome are linked and therefore form ____.

a linkage group

If a dihybrid F1 Drosophila female produces four types of gametes, in a ratio of 1:1:1:1, then the two genes followed in this cross ____.

assort independently

When the alleles of two genes on the X chromosome can be separated due to recombination, the two genes _____.

can appear to assort independently

When scientists want to determine if the observed results from a genetic experiment agree with the predicted results, they often perform a _________ - _________ test.

chi-squared

Chromosomal interference means that a crossover in one region of a chromosome _____.

decreases the probability that another crossover will occur nearby on the same chromosome

Morgan proposed that the likelihood of crossing-over depends on the _________ between two genes.

distance

You are following two X-linked genes in a two-factor cross using Drosophila. If the genes assort independently, an F1 dihybrid female should produce ____.

four types of gametes in a 1:1:1:1 ratio

Consider a three-point testcross involving three linked genes: a, b, and c. The testcross was performed with F1 individuals of the genotype a b c/a+ b+ c+. Among the classes of recombinant progeny obtained were the following: 15 progeny that were a+ b+ c/a b c; 112 progeny that were a+ b c+/a b c; 154 progeny that were a b+ c+/a b c. You can infer that ____.

gene c is between genes a and b

Humans have about 28,000 __________, which are distributed over 23 chromosomes.

genes

A ______ is an allele that could serve as a point of reference in determining if progeny are the result of recombination.

genetic marker

Statistical analysis cannot prove that a is true.

hypothisis

Mendel's law of ______ ______ suggests that a two-factor cross between two individuals heterozygous for two genes should yield a 9:3:3:1 phenotypic ratio among the offspring.

independent assortment

The phenomenon in which the occurrence of a crossover in one region of a chromosome decreases the probability that a second crossover will occur nearby is called chromosomal _______ .

interference

Morgan explained his data by suggesting that crossing-over _____.

is more likely to occur between two genes that are far apart than between two genes that are closer together

When particular alleles of two different genes tend to be inherited together more often than they should according to the law of independent assortment, they are said to exhibit genetic _______

linkage

If gene A is linked to gene B, and B to C and C to D, they are collectively called a _________ _________.

linkage group

A _____ _____ is a diagram that identifies the distances between genes along a chromosome in map units.

linkage map

Based on the ratios between parental and recombinant offspring in the two crosses depicted on the image, you can tell that the white and yellow genes are _____.

linked

When conducting a chi-square analysis on data from a two-factor cross, scientists use statistical analysis to reject or accept the null hypothesis that the genes are not ____________.

linked

If two genes are located on two different chromosomes or are very far apart on the same chromosome, they will exhibit a recombination frequency ____.

of 50%

A cross between a doubly heterozygous female fruit with a w y+/w+ y genotype and a male fly with a w+ y/Y genotype produces several offspring with a w+ y/Y genotype. These flies are considered to be of the _____.

parental type

Cytologically visible abnormalities that make it possible to track chromosome parts from generation to generation are called ______.

physical markers

The _____ rule allows us to calculate the likelihood of a double crossover if we know the individual likelihood of each single crossover.

product

A doubly heterozygous female fruit fly with the genotype w y+/w+ y mates with a w+ y/Y male. Some of the offspring have a w y/Y genotype. These flies are of the ______.

recombinant type

One of the main limitations of using two-point crosses in chromosome mapping is that the order of two genes might be difficult to determine if _____.

some gene pairs are very close together

The null hypothesis for chi-square analysis of a two-factor cross states that ___.

the genes in question are not linked

The closer two genes are to each other, ____.

the less likely it is that a double crossover will take place between them.

Sample size affects the chi-square test because the larger the sample is, _____.

the lower the likelihood is that a deviation in the observed results from the expected results is due to chance

If a chi-square test for goodness of fit performed with data from a two-factor cross returns a p value < 0.01, ______.

the null hypothesis is rejected

In a testcross, if the genes in question are linked, how does the number of recombinant offspring relate to the number of parental offspring?

there are fewer recombinant and nonrecombinant offspring

One map ________ equals 1% recombinant offspring in a testcross.

unit

You are performing a cross involving two X-linked genes in Drosophila. Almost 98% of the males in the F2generation are either w+ y/Y or w y+/Y. What was the most likely genotype of the F1 females?

w+ y/w y+

In fruit flies, one X-linked gene specifies either wild-type red eyes (w+) or mutant white eyes (w). Another X-linked gene specifies either wild-type brown body (y+) or a mutant yellow body (y). Which of the following crosses will produce only females that have the wild-type red eyes and brown bodies?

w+ y/w+ y crossed with w y+/Y

In fruit flies, two of the X-linked white gene alleles are: w+, which specifies red eyes, and w, which confers white eyes. The w+ allele is dominant to the w allele. How would you specify a male with white eyes?

w/Y